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Positive and Negative Subscripts

The first element of an array is element 0. The range of a C++ array is from array[0] to array[size – 1]. However, C++ supports positive and negative subscripts. Negative subscripts must fall within array boundaries; if they do not, the results are unpredictable. The following code shows positive and negative array subscripts:

#include <iostream>
using namespace std;

int main() {
    int intArray[1024];
    for (int i = 0, j = 0; i < 1024; i++)
        intArray[i] = j++;

    cout << intArray[512] << endl;// 512
    int *midArray = &intArray[512];  // pointer to the middle of the array

    cout << midArray[-256] << endl;   // 256

    cout << intArray[-256] << endl; // unpredictable

The negative subscript in the lasta line can produce a run-time error because it points an address 256 bytes lower in memory than the origin of the array. The pointer midArray is initialized to the middle of intArray; it is therefore possible to use both positive and negative array indices on it. Array subscript errors do not generate compile-time errors, but they yield unpredictable results.

The subscript operator is commutative. Therefore, the expressions array[index] and array[array] are guaranteed to be equivalent as long as the subscript operator is not overloaded (see Overloaded Operators). The first form is the most common coding practice, but either works.

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